1. Field of Invention
The present invention relates to seismic exploration and more particularly to the conduct of seismic exploration utilizing vibrator sources for the purpose of introducing seismic energy into the earth in conducting the seismic exploration process.
2. The Prior Art
In the seismic exploration art it has become common to use as a seismic wave source an electrohydraulically controlled vibratory source more simply referred to as a vibrator. Typically a vibrator is vehicle mounted. Upon arrival at a selected location a base plate is lowered into contact with the earth's surface and a portion of the vehicle's weight is applied to the base plate to prevent decoupling of the base plate from the ground during operation. The vibrator applies a force proportional to a swept frequency sine wave or some other function to the earth in a manner well known to the art. With this form of signal, the frequency and phase is controlled to produce a signal with specific repeatable characteristics.
The signal generated by the vibrator must be accurate and repeatable to allow cross-correlation with the system reference signal. If the signal generated by the vibrator does not accurately follow the system reference, the peak-to-sidelobe ratio of the cross-correlation will be low and the sidelobes will extend out from the main lobe causing the data to appear "ringy". If the signal generated by the vibrator varies significantly upon repetition, the seismic data will show variations that may lead to false interpretation. Variations in the sweeps generated by the vibrators can cancel high-frequency information or create sidelobes which vary or create false structures in a seismic section. When multiple vibrators are used simultaneously in a seismic survey to increase the signal to noise ratio, they must all independently generate these signals which are accurate and repeatable. As many as four or more vibrators have been utilized to input seismic energy into earth. Inasmuch as these vibrators are activated simultaneously, it is important that the output energies have very similar phase-frequency characteristics over the length of the sweep wave. it is obvious that if one or more of the vibrators is out of phase with the others, there will take place a cancellation of the energy and defeat the purpose of using multiple vibrators.
Accordingly, efforts have been made in one way or other to test the accuracy and consistency of the vibrator signals and to determine the similarities between the outputs of the various vibrators. This has been accomplished by recording the electronic control signal from one vibrator and the accelerometer outputs from all vibrators by way of an oscillographic camera before the start of exploration each day. The oscillographic camera records the analog electric voltage from the vibrator control signal and the accelerometer output signals. An operator or observer is then required visually to compare the analog traces to determine whether the vibrators are "in phase" with the control signal. If a peak of one trace lines up with the trough of another trace, the vibrators are obviously out of phase. The accelerometer outputs from each vibrator must bear some consistent phase relation to the vibrator control signal. The relation must remain constant or the cross-correlation process will vary. The problem comes about when there is but partial misalignment and the judgment becomes subjective. This relationship is difficult to determine and is poorly tested by this subjective judgment. A small amount of misalignment in phase is acceptable but determining the absolute amount of misalignment is difficult inasmuch as the display of the analog signals may be as much as ten feet long and therefore it becomes extremely difficult to determine whether the similarities are acceptable throughout their total length.